Since recent literature has argued to give up a single explanation covering diverse symptom defining autism-spectrum disorder（ASD）, our study focuses on deficits in social cognition as the target of phenotype in searching cause for ASD. First study revealed that the gray matter volume reductions in several regions important for social cognition were common to monozygotic twins with Asperger's syndrome as compared with healthy matched controls. The findings suggest a contribution of shared genetic factors to underlying the structural abnormalities in ASD. Second study showed that the young healthy females showed greater cooperativeness as well as larger relative global and regional gray matter volumes than the matched males, particularly in the social-brain regions including posterior inferior frontal and anterior medial prefrontal cortices. Moreover, specifically in females, higher cooperativeness was tightly coupled with the larger relative total gray matter volume and more specifically with the regional gray matter volume in most of the regions revealing larger in female sex-dimorphism. These results suggest that sexually-dimorphic factors may affect the neurodevelopmen f these “social-brain”regions, leading to higher cooperativeness in females. The findings may also have an implication for the pathophysiology of autism ; characterized by severe dysfunction in social reciprocity, abnormalities in social-brain, and disproportionately low probability in females. Third study showed a gender specific relationship between a polymorphism of oxytocin-receptor gene and regional gray matter volume of inferior frontal gyrus in healthy young adults. Forth study demonstrates the correlation between smaller-than normal volume of posterior inferior frontal gyrus and worse function of social communication in the males with ASD compared with matched controls. Furthermore, we would like to discuss the possibility of future study examining the relationship between oxytocin-induced enhancement of social cognition and polymorphisms of genes encoding oxytocin-related molecules using neuroimaging as endophenotypes.